Induction mine firing and timing system



Feb. 6, 1962 Filed Jan. 17, 1947 w. R. MALTBY ETAL 3,019,730

INDUCTION MINE FIRING AND TIMING SYSTEM 2 Sheets-Sheet l 4 IIIIIIIIIIIIIIIIII, rzlllyl lllrilllll v v L gwuam bom WELZWZZZZby 1962 w. R. MALTBY ETAL 3,019,730

INDUCTION MINE FIRING AND TIMING SYSTEM 2 Sheets-Sheet 2 Filed Jan. 17, 1947 awn Patented Feb. 6, 1962 INDUCTEGN NHNE FIRlNG AND TlMlNG SYSTEM Wilson R. Maltby, Arlington, Va. {9319 Cedar Lane,

Bethesda, Md), and Robert H. Park, Bernardsville,

NJ. (Corporation Road, Dennis, Mass.)

Filed Jan. 17, 1947, Ser. No. 722,608 18 Claims. (Cl. 102-18) (Granted under Titie 35, US. (lode (1952), sec. 266) This invention relates to a mine firing mechanism of the electromagnetic induction type in which a changing magnetic field is employed to fire the mine and in which the possibility of detonation by mine sweeping operations is extremely remote. More specifically the present invention contemplates the provision of means for rapidly dissipating energy stored in the detcting circuit in response to sweeping operations and thereby preventing repeated actuation of the firing mechanism by single large impulses of sweeping current.

In devices of this character heretofore proposed in which a changing magnetic field is employed for firing a submarine mine, it has been found that an impulse of heavy current applied to an electrical conductor attached to a vessel moving within the vicinity of the mine and insulated from the sea water within which it is immersed except at the furthermost end thereof, causes a magnetic field to be set up within the Water about the conductor of suificient intensity to cause the mine to explode. This method of detonating a mine is referred to herein as mine sweeping, the electrical conductor employed for this purpose is also referred to herein as a sweep wire, and the vessel as a mine sweeper. In mine sweeping of this type very large impulses of sweeping current are sometimes employed with the result that energy is temporarily stored in an induction coil circuit of the type disclosed and which is disposed within the field generated by the sweeping current. This has been found to result in repeated actuation of the detecting circuit and ultimately in sweeping or detonating of such mines.

In the system of the present invention the mine firing mechanism includes certain selecting and time delay elements in combination with the field sensing device whereby the detonation of the mine is efiected only when the magnetic field continues to change for a predetermined period of time such, for example, as the continuing change of magnetic field caused by the movement past the mine of a steel vessel, or any vessel having a considerable amount of magnetic material therein when the mine is disposed adjacent the path of travel of the vessel. By reason of the signal detecting and selecting elements and the control circuit of the firing mechanism, detonation of the mine is assured when a ship passes in the vicinity of the mine. The selecting and time delay features of the circuit provide a high degree of protection against sweeping of the mine by sweeping methods heretofore employed in which the field of a ship is not simulated by the mine sweeper. It has been found that when a sufiiciently large change of magnetic field is suddenly produced in the vicinity of the mine, as for example by producing a large impulse of current in a trailing sweep wire, some of the energy is transferred to and stored in the detecting circuit of the mine firing mechanism for a sufficient time to cause repeated actuation of the detector circuit much in the manner of repeated actuations thereof by a continuously changing field, and sweeping of induction mines heretofore employed or proposed is possible. In accordance with the practice of this invention the energy so stored in the detector circuit by large changes of magnetic field is rapidly dissipated so as to be ineffective to cause repeated actuations of the detector circuit.

One of the objects of the present invention is to provide an improved magnetic influence firing mechanism, selectively responsive to changes in the magnetic field for detonating a mine in response to field changes of predetermined type and sequence, the mechanism being non responsive to changes of the field not occurring in the predetermined sequence.

Another object of the invention is to provide means for rapidly dissipating energy stored in the detector circuit of an induction mine firing mechanism in response to a large change in the field surrounding the detector.

Another object resides in means for preventing storage of eddy currents in the materials surrounding the detector coil and the feeding back thereof into the coil during successive response periods of the mechanism.

Another object is to provide a mine firing control circuit which limits response of the detector circuit to field changes occurring in predetermined ordered sequence.

Another object is to provide means within the firing mechanism for interrupting persisting transient currents within the detector circuit resulting from large and suddenly applied changes of magnetic field.

Another object is to provide an improved firing mechanism operable in response to a predetermined number of changes of magnetic field detected in sequential order, and nonoperable when less than the predetermined number of changes are detected.

Still another object of the invention is to prevent energy stored in the detector circuit during a responsive interval from being transformed into eddy currents in the material surrounding the detector coil and thence back into the coil as the detector circuit is made responsive during a subsequent interval.

Still other objects, advantages and improvements will become apparent from the following description, taken in connection with accompanying drawings, of which:

FIG. 1 is a view partly in section of a submarine mine employing the present invention; and

FIG. 2 is a schematic diagram of the circuit employed in a mine firing mechanism illustrating the present invention.

Referring first to FIG. 1 of the drawings there is shown thereon a submarine mine indicated generally by the numeral 11 having therein and extending substantially throughout the full length thereof a tube 12 which is secured to the mine in any suitable manner such as by the support 14 and the diaphragm 21. The mine is preferably fitted With fins 13 for the purpose of directing the flight of the mine along the intended trajectory. The mine is also provided with pockets l5 and 17 in which are disposed the arming switches, means for operating the arming switches, a booster charge for firing the explosive Within the main cavity of the mine, a detonator for firing the booster charge, an extender mechanism for placing the detonator within the booster charge after the mine has been submerged in a predetermined minimum depth of water, and the necessary electrical connections from the mine firing mechanism for firing the detonator. Tubes 16 are provided within the main cavity of the mine connecting the pockets 15 and 17 with the after compartment of the mine for carrying cable connections to the control circuits of the firing mechanism.

The tube 12 in accordance with this invention is composed of a nonmagnetic material of high electrical resistance such, for example, as Bakelite, stainless steel and the like. This tube is secured within the mine in a fixed position preferably parallel to the longitudinal axis of the mine whereby a maximum length of tube may be employed and the orientation of the coil tube is determined. This coil tube is of such dimensions as to fit snugly about the induction coil 24 contained therein so as to rigidly support the coil throughout the length thereof.

The induction coil or search coil 24 is shown assembled in the tube 12 and is supported laterally by the walls of the tube and is cushioned at the forward end thereof against shocks incident to launching the mine by the cushion 19 which is arranged within the forward end 'of the tube. Within the coil 24 and throughout the length thereof is disposeda core 25 of magnetic material, such for example as Permalloy, possesses the, characteristic of high initial permeability when disposed within weak magnetic fields such as are normally encountered at the surface of the earth. The ends. of the core 25 are preferably provided with magnetic flanges for the purpose of concentrating additional flux throughout the length of the core in order to increase the amount and uniformity of the flux change which it is desired to detect by means ofthe coil 24 and its'associated detector circuit.

Within the after chamber of the mine the firing mechanism housing, indicated by the numeral 27, is shown within a .shock mounting suitable for absorbing the initial peak of the shock encountered as the mine strikes the water. Within the mechanism housing as shown in FIG. 2 is the relay D, the actuating coil thereof 22, the

connecting conductors 23and parallel switch contacts 26,

the firing mechanism herein disclosed in readiness for operation in response to changing magnetic fields. One of the arming switches is 'anormally open pair of contacts, designated 2 in FIG. 2, which is closed upon arming of the mine thereby connecting the battery 32 to the mechanism for operating the control circuits of themechanism.

The operation of the firing mechanism will best be understood by reference to FIG.'2 inwhich the components are shown diagrammatically connected in operative relationship to each other. The coil 24 is shown disposed within the tube 12 and is shown with itsleads 23 connected to the operate coil 22 of relay D, one of the leads 23 being connected to the operate coil through parallel contacts 26 of a relay Q. The contacts 26 are normally in a closed position and completethe electrical circuit through the coil 24, the operate coil 22 and the leads 23. This circuit is isolated electrically from the remainder of the mechanism and is herein referredto as the detector circuit. The relay Q is of the. quick-acting and slow-release type by virtue of the copper sleeve disposed about the heel end of the relay core.

The relay D may conveniently be of the: type known in the trade as a Sensitrol relay and is designed to have high sensitivity so as to operate in response to very small currents passing through the coil 22, for example, one to ten microamperes. magnetic in order to firmly engage the armature 35 when contact is made in response to passage of current of either sign through the coil 22. This contact between the armature and one of theifixed contacts, when established, is not interrupted until the armature 35 is mechanically restored to a central position between the contacts. The resetting of the armature is accomplished by means of a resetting solenoid or magnet DR which forms a part of the relay D. The solenoid is actuated as hereinafter described by closing of the circuit from DR to the positive side of the battery, the electrical connection to negative battery being completed through the arming switch when the mine is armed. The coil of the relay Q is connected by the conductors 37 and 38 across the winding of the solenoid DR in parallel and therefore operates simultaneously with the operation of DR.

-Mounted in any convenient manner adjacent to the relay D is a relay P of the quick-acting and slow-release type and adapted to close normally open'contacts thereon by means of the armature 36. The coil of the relay P is'energized whenever the contacts of the relay D are closed either at 33 or 34 whereby positive battery is supplied to the coil of P. Negative battery is connected as hereinbefore stated through the arming switch, the conductor 39 and thence to the coil of P.

Also mounted within the mechanism in any convenient manner is a'relay TD1 which is of the quick-acting and slow-release type and having normally open contacts which are closed by the contact 41 when the coil of relay TD1 is actuated. The delayed opening of the contacts of TD-l may conveniently be accomplished by a clock escapernent mechanism or by any other suitable means which will provide a suitable relay such as, for example, three seconds, which interval will be used by way of illustration in this description. The relay TD1 is operated by connecting negativebattery to the coil thereof by way of conductor 39, the contacts of armature 36 and the conductor 42, the connection to positive battery being by way of conductor 43. The contact 41 is also connected to positive battery by conductor 43. Upon actuation of the relay TD-1 positive battery is applied to the solenoid DR of relay D and to the relay Q, causing DR to be actuated and the detector circuit to be broken at 26. DR and Q remain operated'while the contacts of TDE are closed. At the expiration of the TD-l delay interval the contacts open and the coils of the magnet DR and Q are deenergized. DR releases immediately and Q releases after a short delay by virtue of the copper sleeve at the heel end thereof, thereby providing a sufficient interval before the detector circuit is rendered effective so that other transients in the control circuit will be ineffective to feed back into the detector circuit. 7

From the foregoing description it will be apparent that actuation'of relay D in response to currents generated in coil 24, causes actuation of relay P followed by actuation of relay T D1, and immediately thereafter the resetting of relay D and the'breaking of the detector circuit at the contacts 26 whereby further currents are prevented from passing through the coil 22 until relay Q has been released. At the same time the armature 35 is restored'to itsinitial position in readiness for detection of further changes in the ambient magnetic field. The purpose of breaking the detector circuit by the contacts 26 will be apparent as the description proceeds.

A selector switch B is also mounted within the mechanism comprising an actuating coil STPB, a wiping contact arm -44, which is advanceable in steps to a plurality of contacts illustrated in PEG. 2 by the numerals The fixed contacts 33 and 34 are 1 to 10 thereon, a resetting magnet RS-B for restoring the contact arm 44 to the normal position as shown, and the oil-normal switch ON-B. This last named switch is normally open and is closed whenever the contact arm' 44 is not in the normal home position illustrated in FIG. 2, thereby completing a circuit through the operate coil of the TD-2 hereinafter described.

A second selector switch A is also mounted within the mechanism and is generally similar to the first selector switch except that two banks of contacts are provided as illustrated at 1-A and 2A. This switch is actuated by the magnet STPA thereon and may be reset to the initial position thereof as shown, by actuation of the magnet RS-A thereon, the circuit for which may be closed by closing the switch 1 in the arming mechanism located preferably so as to be actuated when the arming process has been partially performed. A selector switch SS is mounted within the mechanism in the vicinity of the second selector switch and has terminals 1, '2, 3, '4, 5, 6, 7, 8, thereof connected respectively to terminals 2, 3, 4, 5, 6, 7, 8, 9 of bank 2-A of the selector switch A. SS includes a sliding contact arm 45 adapted to connect predetermined terminals of SS with conductor 46 and the detonator 47.

Also included in the mechanism is a relay TD-Z which includes normally open contacts which are closed when the contacts 48 thereof are closed by the coil or magnet of the relay TD-2. This relay is of the slow operate type and may convel- 'ently provide any suitable delay in the closing of the contacts 43 thereof in consequence of operation of a cioclt escapement mechanism energized by the magnet of the relay. When the contacts of the Til-2 close selector switch B is reset by operation of RS-B and the contacts ON-B are reopened deenergizing the coil of the Til-2. Whenever the coil is deenergized this relay returns quickly to the normal position with the contacts thereof open. This terrm'nates a cycle of operation and leaves the mechanism in its original condition unless the switch A has been operated.

it will be seen from the above description taken in connection with PEG. 2 that the control circuit of the mechanism is adapted to respond selectively to a series of changes in the magnetic field occuring in time spaced relationship. it will also be apparent that the occurence of magnetic field changes in the vicinity of the detector circuit is inefiective to actuate the mechanism during the intervals of time when the reset magnet DR is actuated, or when the relay Q is actuated such that the detector circuit is inoperative.

For the purpose of this discussion the interval during which the detector circuit is held open by relay Q will be assumed to be approximately 3 seconds. This is referred to as the dead interval. By way of example it will be assumed that the TD2 operates in approximately seconds, thus providing a live period of 10 seconds following the first operation of relay D, in any cycle, during which additional advances of the wiper 44 may be performed before closure of contacts 48 cause resetting of 44. Let it be further assumed that switches T84 and TS-Z are open and TS3 is closed so that the fourth actuation of switch B connects conductors 3 and .9 through 44, and that the mine has been armed with switch 1 open, switch 2 closed and selector switches A and B having their contact arms at positions 1 and 0, respectively. It will also be assumed that switch SS is adjusted so that the detonator will be fired after the first actuation of selector switch A through switch 2, conductor 3?, contact 2 of bank 2-A of selector switch A, contact 1 of SS, conductor 46, detonator 47 and conductor 43 to battery. This settin of SS is referred to as a setting to fire on the first shipcount.

With the mine thus armed any change occurring in the magnetic field in the vicinity of the mine is detected by the coil 24 in which an EMF. is generated in accordance with the magnitude and rate of change of the field. This causes current to flow through. conductors 23, contacts 26 and coil 22. If the field change is of sufficient magnitude and rate of change the relay D is operated and the moving armature 35 makes contact with 33 or 34 in accordance with the direction of the EMF. produced in the coil 24. When contact is made between 35 and 33 or 34 the coil of relay P is actuated, the contacts of armature 36 are closed, the TD-l magnet is operated, thereby closing contacts 41 and the magnet STPB is operated causing 44 to advance to contact 1 and close contacts ON-B. The closing of contacts ON-B energizes the magnet of TD.?. by way of conductors 39 and 51, the arm of switch A, conductor 53 and the positive battery connection 43. Since the relay TD2 is of the slow-operate type, contacts -48 are not immediately closed. The relay TD-Il is of a slowrelease type but operates rapidly to place a positive battery on conductor 54 whereby magnet DR and relay Q are actuated. Operation of DR opens the contacts of relay D thereby deenergizing the coil of relay P. Since P is a slow-release relay, sufficient time is provided to 6 assure that STP-B, TD-1 and Q have all operated prior to opening of contacts 36.

Since the contacts 41 reopen about 3 seconds after operation of relay P the magnet DR and the relay Q release approximately 3 seconds after actuation and the armature 35 of relay D is thus placed in readiness for a second actuation thereof by the detector circuit. A fraction of a second thereafter relay Q releases and contacts 26 close to complete the circuit from the coil 24 to the coil 22.. The detector circuit is now in readiness to respond to a further change in magnetic field, and the first dead interval during which changing magnetic conditions were ineffective to operate relay D has terminated.

During this dead interval the relay TDZ has been operating and is now advanced about 3 seconds of the 10 seconds required to close contacts v3. If a second change in magnetic field is detected and the relay D is again operated wiper arm 44 of the switch STP-B is advanced to position 2, corresponding to the second change of field detected. TD1, DR and Q are actuated in the same manner as in the case of the first operation of relay D. When relay Q again releases to reclose the detector circuit 3 seconds after the second operation of relay D the TD-2 will have advanced 6 seconds toward closure of the contacts 48, or more, depending upon the delay in detection of the second changing field after the first reclosure of contacts 26. If a third change in magnetic field is promptly detected by the detector circuit the wiper 44 will be advanced to contacts 3 in the same manner as before and a third dead interval is initiated by operation of magnet DR and relay Q. This third dead interval terminates 9 seconds after the first actuation of relay Q, or more, depending on the delay in each instance in operation of D. If each actuation of relay D has occurred promptly after closing of contacts 26 the detector circuit will be ready for a fourth actuation prior to the closures of contacts 48 of relay TD-Z. if the fourth actuation of relay D is received within 10 seconds of the first actuation, that is before contacts 48 are closed 44 will be advanced to position 4, thereby completing a circuit from negative battery by way of conductor 39, wiper 44, terminal 4, switch TS-li, conductor 49, actuating magnet SPT-A and conductor 43. When SPT-A operates the wiper of switch A advances to position 2 and completes a circuit from negative battery by way of conductor 39, bank ZA of switch A, the wiper and terminal 2 of bank 2A, switch SS, the conductor 46, detonator 47 and conductor 43 to positive battery. The mine is thus fired when 4 changes in field are detected at regular intervals controlled by T D-1 and Within a period of time controlled by TD-2.

If the fourth actuation of relay D does not occur prior to the termination of the operate time of relay TD-2 the contacts 48 thereof close and complete a circuit from negative battery by way of conductor 39, contacts 48, conductor 52, reset magnet RS-B and conductor 43 to positive battery. When magnet RSB operates the wiper 44 is released and returns immediately to the home position 0 from which it started, and contacts ON-B are opened. Opening of contacts ON-B deenergizes the magnet of TD2 and the control circuits are promptly restored to the initial position as though no actuation of the detector circuit had occurred. The control circuit is therefore effective to select only such changes of field as occur at regular intervals, in the above instance about 3 seconds, and reject all other changes of field which may have occurred during the actuation of the relay TD2. It is apparent that by selecting other time intervals for delayed opening of contacts 41 and closing of contacts 48 that the circuit may be made responsive to any desired frequency of changes in field. The firing mechanism theerfore operates to select or reject field changes depending upon whether such field changes occur in predetermined spaced relation. Any desired number of changes of field may be registered prior to actuation of stepping switch A by selection of the number of connections left open on selector switch B, as for instance by opening or closing switches TS-l, TS-2, TS-3, etc.

When a sensitive relay such as D is connected to a high inductance search coil, such as 24, employing therein a flux concentrating core, such as 25, the circuit has a large time constant. Any sudden change in the ambient field produces a change in the flux through the core and a current in the detector circuit. The energy thus transferred to coil 22 is utilized in causing rotation of coil 22 and of the armature 35 which is secured thereto. Energy in the detector circuit is thus absorbed by movement of the moving coil until the armature 35 is caused to touch fixed contact 33, or 34 as the case may be. If the available energy has not been substantially absorbed by this rotation the remaining energy is stored in the circuit temporarily and does not immediately dissipate as thermal energy. It has been found that when very large changes in the ambient field occur the energy thus stored in the detector circuit is sufficient in magnitude and remains in storage for a sufiicient period of time to cause the relay D to repeatedly operate upon release of the reset magnet DR several seconds after the first operation thereof when no further change in the ambient field has occurred. For this reason a single large change in the field has been found in some instances to operate the relay D three or four times extending over a period of 6 to 10 seconds after the first operation. This storage of energy in the detecting circuit depends on the abruptness and magnitude of the field change and is unimportant in changes such as are produced by passing ships.

Whereas the firing mechanism herein disclosed is effective to select changes of magnetic field occurring in regular order within a predetermined period of time and reject changes of magnetic field occurring at other than the regular responsive periods provided, when such changes of field are comparable in magnitude and rate of change with the field changes resulting for the passage of a vessel, the same degree of selection and rejection is not obtained when the changes of ambient field are of sufficient magnitude and rate to result in storage of energy over a period of several seconds. This invention provides means for confining the response of the detonator circuit to the responsive periods set up by the timing circuits of the mechanism so as to make large artificially produced field changes inefiective to sweep the mines.

To accomplish this purpose the relay Q operates to interrupt or open the detector circuit at the contacts 26 during the intervals of time when the armature 35 is restrained from movement by the reset magnet DR. When these contacts are open no current can fiow in the detector circuit and the transient energy stored 'within the circuit decays exponentially at a rapid rate. This decay in energy has been found for practical purposes to reduce the stored energy to a negligible quantity in a small fraction of the time otherwise required. It is convenient for some purposes to have a shunting resistance across the contacts 26 whereby the operation of the relay Q functions to increase the resistance of the detector circuit for a predetermined time rather than to break the circuit.

I For this purpose a resistance is preferred which is high with respect to the resistance of the circuit in the absence of the resistance so that a relative large change is made in the circuit resistance when the relay operates, whereby a rapid decay in stored energy is achieved.

In FIG. 2 the contacts 26 are shown to be arranged with three sets of contacts in parallel. Minute particles of dust on foreign material sometimes causesa single set of contacts to fail to make connection to permit the passage of currents of the order here available, such as 1 to 10 microamperes, especially when the impressed volt age does not exceed a few micro volts. The use of multiple contacts in parallel reduces this possibility of failure and furthermore assures a lower resistance connection than when single contacts are employed.

8 The relay Q is provided with a copper sleeve in order to delay the release of the relay for an appreciable time after interruption of the energizing current thereto. This delay time is electromagnetic in nature and depends on circulating currents induced in the copper sleeve when the energizing current is interrupted, and is a phenomenon well known in the art. Th s additional delay in reclosing of contacts 26 is provided in order that actuating currents within the various relays and switches of the mechanism and the resulting fields locally generated during each cycle of operations initiated by each operation of relay D will be dissipated prior to the closure of contacts 26. This feature is provided to assure that local changing fields in the mechanism will be ineffective to induce currents in the coil 24 at any time during which the detector circuit is responsive. Magnetic feedback from the mechanism to the detector circuit is thus minimized.

When used in a mine of the character disclosed a coil of the type required for detection of passing ships has previously been mounted in a heavy brass or bronze tube in order to support the relatively long and slender coil against deformation or damage resulting from handling or dropping the mine. This tube must be of nonmagnetic material in order that there will be no shunting of the magnetic field it is desired to concentrate within the core 25. Furthermore the tube is normally constructed with walls sufiiciently heavy to form a strong and rigid support for the coil because of the fact that the poor material is easily damaged by bending. This tube closely surrounds the coil so that the coil fits snugly therein.

When means is provided for interrupting the detector circuit for an interval following each actuation of relay D, or for greatly increasing the resistance thereof by opening the contacts 26, it has been found that an appreciable portion of the energy previously stored within the detector circuit is transferred from that circuit to the walls of the tube surrounding the coil. Upon interrupting the detector circuit the brass tube surrounding the coil operates much in fashion of a single turn, low resistance, shorted transformer secondary of very low resistance. The rapid decay of current in the detector circuit, therefore, induces a much larger current in the low resistance single turn secondary, i.e., the surrounding tube. The energy thus transferred from the detector circuit to the coil tube gradually dissipates as thermal energy in accordance with the square of the induced current times the resistance of the secondary circuit. This secondary current has been found to persist for a suflicient time after interrupting the circuit at 26 to cause in some instances where the interrupted current was relatively large, a feedback into the detector circuit of suificient magnitude to cause a second or a third operation of the relay D without further change in the ambient field after the initial operation of relay D. It may therefore be seen that the low resistance material normally employed in the nonmagnetic tubes of the sort previously used for coil tubing for coil supports results in an action similar to that produced by the copper sleeve employed on the heel end of the tube. This serves as a second means for restoring energy so that when the flow of current in the detector circuit is prevented by introduction of a large resistance, this energy is transformed to the coil tube and a part of it again transformed into the detector circuit as soon as the low resistance detector circuit is reestablished. In accordance with this invention 'a means has been found to substantially prevent this transformer effect by the use of a high resistance material for the coil tube. The various dielectric materials such as Bakelite, Formica or other synthetic laminated materials possessing high strength may be employed to prevent the transformer action hereinbefore described. However such dielectric materials can not be welded to the mine casing and are very difiicult and expensive to use because of the difiiculty and cost of anchoring such nomnetallic materials to the mine case with sufficient rigidity. Stainless steel has been found to possess sufiicient strength when used in a relative thin walled tubing to adequately support the coil within the mine case. This material is easily welded at both ends to the mine case. Furthermore the specific resistance of stainless steel is sufficiently high when used in a thin walled tubing to largely prevent the transformer action, and to rapidly dissipate energy which may have been temporarily stored therein by the relatively small degree of transformer action which does occur.

The use of a high resistance tube surrounding the coil thus cooperates with the circuit interrupting feature as hereinbefore described and supplements the action thereof to cause rapid decay of the energy stored within the detector circuit so that very large and sudden changes of ambient field are ineffective to cause more than a single operation of relay D resulting from each detected change of the field. The combined operation of the relay Q for a predetermined time and the dissipation of energy within the high resistance tube surrounding the coil are effective in preventing carry-over of energy from one responsive period of the detector circuit to the next succeeding responsive period. When the contacts 26 are reclosed to initiate a succeeding responsive period the means for dissipating energy within the detector circuit is rendered inoperative and remains inoperative until relay D has again operated. Since the stainless steel employed in the coil tube is nonmagnetic there is little or no magnetic shunting of the search coil.

In this manner applicants achieve a practical solution to the problem of circulating currents which have heretofore resulted in repeated operations of relay D when the ambient field is changed abruptly or by an amount several hundred times that sufficient to actuate relay D. In this manner easy sweeping of magnetic mines is prevented even though very large field changes are produced in the vicinity of the mine by the sweeping apparatus. A much more efiicient and reliable mine is made possible by the use of this invention.

While the invention has been described with reference to a particular example which gives satisfactory results, it will be apparent to those skilled in the art to which the invention pertains, after understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention and it is intended, therefore, in the appended claims to cover all such changes and modifications.

The invention herein described and claimed may be manufactured and used by or for the Government of the UnitedStates of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In an induction control mechanism adapted to produce an electrical signal in response to a predetermined number of changes in the ambient field occurring in a predetermined sequential order, a normally closed detector circuit comprising a sensitive detector coil and a sensitive relay adapted to be actuated by currents generated in said coil in response to said changes, and means controlled by said relay for interrupting said circuit for a predetermined period of time following each actuation of the relay.

2. In an induction mine firing mechanism adapted to fire a mine in response to a predetermined number of changes in the ambient field occurring in regular sequence within a predetermined period of time, a detector circuit comprising a sensitive relay responsive to signals in said circuit, a detector coil included in the circuit for generating said signals in response to said changes in the field, a timing device controlled by the sensitive relay, and a second relay controlled by the timing device and having means adapted to increase the impedance of the detector circuit for an interval following each actuation of the sensitive relay whereby transient currents in the detector i0 circuit are quickly subdued after each operation of the sensitive relay.

3. In an induction mine firing mechanism adapted to fire a mine in response to a series of changes in the ambient field, a detector circuit comprising an induction coil and a sensitive relay, timing and switching means adapted to render the detector circuit responsive only during predetermined periods of time, and circuit interrupting means within the detector circuit adapted to be operated by said timing and switching means after each of said predetermined periods of time whereby energy stored in the detector circuit during one response period thereof is prevented from repeatedly operating the circuit during subsequent responsive periods thereof.

4. In an induction controlled firing mechanism for a mine, an induction coil, a relay operatively connected to the coil, electromechanical timing and switching means for mechanically rendering the relay nonresponsive to signals generated in said coil during a series of predetermined intervals while leaving the relay responsive during intervening intervals, a switch mechanism controlled by the timing and switching means and electrically connected to interrupt the circuit from the induction coil to the relay during substantially said predetermined intervals, and means for firing a detonator when a signal has been generated in the induction coil during each of said intervening intervals.

5. In a submarine mine of the character disclosed arranged within a magnetic field, a detector circuit responsive to changes in said magnetic field including a search coil and a relay connected thereto, means controlled by said relay for counting signals corresponding to said changes in said field detected by the detector circuit, means effective when the first of said changes has been detected for measuring a predetermined period of time, means controlled by the relay for interrupting the detecting circuit for a predetermined interval after each operation thereof thereby to cause rapid decay of transient currents therein, and means controlled by said counting means for firing the mine when a predetermined number of changes of field have been detected within said predetermined period of time.

6. In a submarine mine of the character disclosed arranged Within a magnetic field, an induction coil for detecting changing conditions within said field, a relay connected to said induction coil and operable in response to currents generated in the coil by said changing conditions, means controlled by said relay for counting signals corresponding to the changes in the field detected by said coil, means responsive to operation of the relay when the first of said conditions has been detected for measuring a predetermined period of time, means controlled by said counting means for causing said mine to explode when a predetermined number of signals have been counted within a predetermined period of time, and means controlled by said relay for opening the detector circuit for a predetermined interval after each operation of the relay whereby transient currents in the induction coil are rapidly dissipated.

7. In a submarine mine of the character disclosed arranged within a magnetic field, an inductive detector circuit responsive to changes in said field comprising a search coil operatively connected to a sensitive relay, means for preventing operation of said relay during predetermined intervals following each operation of the relay, means controlled by the relay for counting signals corresponding to successive operations of the relay, means effective after the first operation of the relay for measuring a predetermined period of time, means effective at the expiration of said period of time for restoring the counting means to the initial setting thereof, energy dissipating means effective after each operation of the relay for preventing the repeated counting of signals when only one change in the field has been detected, and means effective for firing the mine when the relay has caused a predetermined number of signals to be counted within the said period of time.

8. In a submarine mine of the character disclosed arranged within a magnetic field, a detecting circuit including a coil responsive to changing conditions within said field and a relay connected to said coil and operable in response to currents generated in the coil by said changing conditions, means controlled by said relay for counting signals corresponding to each operation of the relay, means controlled by said counting means for measuring atpredeterrnined period of time, means controlled by said measuring means for restoring the counting means to the initial condition thereof at the terminationof said predetermined periods, means controlled by the relay for mechanically restraining operation of the relay for an interval following each operation thereof, means controlled by the relay for stopping the flow of transient energy in the detecting circuit during the interval in which the relay is restrained after each operation thereof, and means for firing the mine when a predetermined number of operations of the relay occur within the predetermined period.

9. In a submarine mine of the character disclosed arranged within a magnetic field, a detecting circuit including a coil responsive to changes within said field and a relay connected to said coil and operable in response to currents generated in the coil by'said changes within the field, means controlled by said relay for counting signals corresponding to each operation of the relay, mean controlled by said counting means for measuring a predetermined period of time, means controlled by said measuring means for restoring the counting means to the initial condition thereof at the termination of said predetermined period, means controlled by the relay for mechanically restraining operation of the relay for an interval following each operation thereof, and a second relay adapted to operate simultaneously with said mechanical restraining means and having normally closed contacts connected in series with the detecting circuit whereby the detecting circuit is interrupted when said second relay operates.

10. In a submarine mine of the character disclosed arranged within a magnetic field, an induction coil for detecting changing conditions within said field, a relay connected to said induction coil and operable in response to currents generated in the coil by said changing conditions, means controlled by said relay for counting signals corresponding to each operation of the relay, means controlled by said counting means for measuring a predetermined period of time, means controlled by said measuring means for restoring the counting means to the initial condition thereof at the termination of said predetermined period, means controlled by the relay for mechanically restraining operation of the relay for an interval following each operation thereof, means controlled by the relay for interrupting during said interval the circuit from the induction coil to the relay following each operation thereof whereby transient energy stored in the induction coil is rapidly dissipated, and means effective for firing the mine when a predetermined number of relay operations have been counted in the predetermined period.

11. In a submarine mine of the character disclosed arranged within a mangetic field, an inductive search coil enclosed within a nonmagnetic tube of high specific resistance, relay connected to said coil and operable in response to currents generated in the coil by changes in said magnetic field, an interval measuring device controlled by said relay, means controlled by said relay for counting signals corresponding to each operation of the relay, means controlled by said counting means for measuring a predetermined period of time, means controlled by said measuring means for restoring the counting means to the initial condition thereof at the termination of said predetermined period, means controlled by said interval measuring device for mechanically resetting and restraining further operation of the relay for an interval following each operation thereof, a circuit interrupting switch in series withsaid coil and relay and controlled by the interval measuring device for stopping continuous flow of current in the coil and relay circuit during said interval during which the relay is mechanically restrained, and means for firing the mine when the counting means has counted a predetermined number'of signals during said predetermined interval of time.

12. In a submarine mine of the character disclosed arranged within a magnetic field, a detector circuit including a highly inductive pickup coil and a relay operatively connected thereto, means controlled by the relay for counting a series of signals generated by said pickup coil during successive predetermined intervals, circuit means for causing the detector circuit to be responsive and nonresponsive alternately during said intervals, means for interrupting the detector circuit during each of said nonresponsive intervals whereby the energy stored in the detector circuit is rapidly dissipated, a tubular supporting structure for said coil and composed of high resistance nonmagnetic material whereby currents induced therein by the interruption of the detector circuit are rapidly dissipated, and means for firing the mine when a predetermined number of signals are counted within said predetermined responsive intervals.

13. In a submarine mine of the character disclosed arranged within amangetic field, a detector circuit responsive to changes in said magnetic field including a search coil and a relay connected thereto, means controlled by said relay for counting signals corresponding to said changes infield detected by the detector circuit, means effective when the first of said changes has been detected for measuring a predetermined period of time, means controlled by the relay for interrupting the detecting circuit for a predetermined interval after each operation thereof thereby to cause rapid decay of transient currents therein, a tubular support for said coil composed of a nonmagnetic alloy and secured longitudinally within said mine, said alloy being of relatively high electrical resistance, whereby circulating currents induced therein by said interruption of the detector circuit are quickly damped out, and means controlled by said counting means for firing the mine when a predetermined number of changes of field have been detected within said predetermined period of time. 7

14. In a submarine mine of the character disclosed arranged within a magnetic field, a detecting circuit including a coil responsive to changing conditions within said field and a relay connected to said coil and operable in response to currents generated in the coil by said changing conditions, means controlled by said relay for counting signals corresponding to each operation of the relay, means controlled by said counting means for measuring a predetermined period of time, means controlled by said measuring means for restoring the counting means to the initial condition thereof at the termination of said predetermined period, means controled by the relay for mechanically restraining operation of the relay for an interval following each operation thereof, means controlled by the relay for stopping the flow of currents in the detecting circuit during the interval in which the relay is restrained after each operation thereof, a supporting structure enclosing said coil within said mine and composed of nonmagnetic material possessing high electrical resistance whereby currents induced in said structure by stopping the fiow of current in the detecting circuit are rapidly dissipated, and means for firing the mine when a predetermined number of operations of the relay occur within the predetermined period. 7

15. In a marine mine firing mechanism of the character disclosed arranged within a magnetic field, the combination of an induction detector circuit responsive to changes in said field and comprising a sensitive relay and a search coil operatively connected thereto for operating the relay in response to current generated in the coil by said field changes, means for preventing spurious operation of said relay in response to energy stored in said circuit during predetermined spaced intervals following operations thereof in response to said field changes respectively, step-bystep switch means operatively connected to said relay for moving the switch means from an initial position forward one step corresponding to each successive operation of the relay, means effective after each operation of the relay for measuring a predetermined period of time, means efiective at the expiration of said predetermined period of time for restoring the switch means to the initial position thereof, and means for firing said mine when said switch means has taken a predetermined number of steps within said predetermined period of time,

16. In a search coil of a character disclosed for generating current therein in response to a plurality of changes in the magnetic field adjacent thereto comprising, in combination, an elongated metallic cylinder of magnetic material having high initial permeability in weak magnetic fields, a coil of electroconducting wire having a relatively large number of turns of wire wrapped about said cylinder, a non-magnetic high resistance metallic tube closely sleeved about said coil for converting the energy stored in said coil to heat energy, and a tube 01' insulating and heat dissipating material disposed between said metallic tube and said coil for rapidly dissipating said heat energy at the expiration of a change in said magnetic field.

17. In a search coil of the character disclosed for intermittently generating electrical signals therein in response to successive changes in the magnetic field adjacent thereto comprising, in combination, an elongated rod of magnetic material having high initial permeability when disposed within weak magnetic fields, a coil of electroconducting wire having a large number of turns sleeved about said rod, and a metallic, non-magnetic tube of high elec- 14 trical resistance sleeved about said coil and electrically insulated therefrom for preventing storage of eddy currents within said tube and the resulting feeding back therefrom into the coil between successive changes in .said magnetic field.

18. In a mine firing mechanism of the character disclosed for firing the mine in response to a predetermined number of changes in the ambient field occurring in predetermined sequential order comprising, in combination, a normally closed detecting circuit including a sensitive relay adapted to be actuated in response to currents generated in said circuit, an elongated rod of magnetic material having high initial permeability when disposed within weak magnetic fields, a coil of wire also included in said circuit and having a large number of turns of wire wound about said rod, means controlled by said relay for interrupting said circuit for a predetermined period of time following each actuation of the relay, a metallic tube of non-magnetic high resistance material arranged in inductive relation with respect to said coil and electrically insulated therefrom for rapidly dissipating eddy currents in said tube during said predetermined period, means operatively connected to said relay for counting the number of operations thereof, and means operatively connected to said counting means for firing the mine when the counting means has counted a predetermined number of operations of said relay. 7

References Cited in the file of this patent UNITED STATES PATENTS 1,698,812 Gardner Ian. 15, 1929 2,131,164 Chauveau Sept. 27, 1938 2,141,343 Campbell Dec. 27, 1938 2,277,000 Bingley Mar. 17, 1942 2,400,549 Glennon et al May 21, 1946 

